1. Field of the Invention
The present invention relates to a moistening water supply apparatus for a printing press which is used in an offset press to supply moistening water to the plate cylinder.
2. Description of the related Art
In general, in an offset press, an oil ink is coated and applied in an even thickness by an inking roller on and to the surface of a printing cylinder on which lipophilic printing images such as characters, patterns, and the like to be printed are formed together with hydrophilic non-image portions. The characters and the like on the printing cylinder are transferred to a rubber cylinder, and they are thereafter transferred to printing paper. Prior to the inking of the cylinder plate, water is supplied to the hydrophilic portions, i.e., the non-image portions of the cylinder plate so that these portions will not be inked during the subsequent inking process. The supply of water is normally effected from a water draw-up roller partially dipped in water in a moistening water tank, through a water transfer roller which is a rubber roller, then through a water smoothing roller formed of a metal, finally to a moistening roller of which at least the surface portion is formed of a rubber layer.
A rubber, which is used to form such a water transfer roller or a moistening roller, is inherently a strongly lipophilic material. With such a moistening roller, therefore, a phenomenon known as ink catching occurs, in which part of the ink which has failed to transfer from the printing cylinder to the rubber cylinder moves to the moistening roller. When a certain amount of ink has adhered to the moistening roller in this way, the ink separates from the moistening roller, and moves, through the water smoothing roller, to the water transfer roller, hence, to the water draw-up roller as well, finally reaching the moistening water tank. During this movement of the ink, an ink catching phenomenon also occurs in the water transfer roller which is, similarly to the moistening roller, a rubber roller.
When the above-described ink catching phenomenon occurs in the moistening water supply rollers, particularly, in the water transfer roller, this leads to various problems. For instance, the water repellency of the ink causes a shortage in the amount of water supplied to the water transfer roller, hence, a shortage in the amount of water received by the moistening water. Also, the amount of water may vary in the widthwise (i.e., axial) direction. Such shortages or variations in the amount of water lead to inadequate reproduction of the printing images and the non-image portions on the plate, which in turn causes the formation of blurs, or a degradation in the printing quality. Thus, it has been necessary to apply some suitable measures.
To meet this requirement, an apparatus such as that shown in FIG. 2 has been proposed. As shown in FIG. 2, a printing cylinder 1 is disposed in contact with a rubber cylinder 3 for transferring a predetermined printing image to printing paper 2, and also in contact with an inking roller 11 of an ink supply apparatus 10. The printing cylinder 1 is also in contact with, at its position upstream of the position of its contact with the inking roller 11, a moistening roller 26 of a moistening water supply apparatus. The moistening roller 26 is supplied with water in the following manner. A part of water stored in the moistening water tank 21 is drawn up by a water draw-up roller 22. A part of the drawn-up water is squeezed from the draw-up roller 22 by a rider roller 23. The remaining water on the roller 22 is supplied through a water transfer roller 24 and a water smoothing roller 25 to the moistening roller 26. A bridge roller 31 is disposed between and in contact with the moistening roller 26 and the inking roller 11, so that ink adhered to the moistening roller 26 is returned by the bridge roller 31 to the inking roller 11.
However, the apparatus shown in FIG. 2 fails to overcome the problem that ink separated from the moistening roller 26 moves through the water smoothing roller 25 to the water transfer roller 24.
The ink-catching phenomenon in a moistening water supply apparatus is not solely attributable to the lipophilic property of a rubber material used to form a moistening roller, but it is also related to the lipophilic property of a rubber material used to form a water transfer roller, which is also a rubber roller, and which is positioned upstream of the moistening roller. However, no specific report has hitherto been made concerning the prevention of the ink-catching phenomenon as viewed in connection with the relationship between the moistening roller and a rubber roller positioned upstream of the moistening roller.